The general function of a heat exchanger, as the term is commonly understood, is to transfer heat from one fluid medium to another. The fluids may be liquid or gas, or a combination of these. Heat exchanger applications include radiators, convectors, coolers, heaters, evaporators, and condensers. This list is by no means exhaustive. The configuration of heat exchangers generally has developed along two lines, liquid to liquid and secondly, liquid to gas or air. Considering the latter, liquid to air, or vice versa, the flowing liquid medium is usually within the metal conduits, and the exterior surface of the conduit has been maximized for greater exposure to the air or gas fluid.
The present state of the art most generally accepts the extension or increase of the outer surface of the conduit containing the primary liquid by mechanically changing its contour, or by the attachment of additional surfaces to it. Additionally, it is well established that the velocity of movement of either or both fluids changes the rate of heat transfer, in relationship already established. In the application of heat exchangers used as evaporators and condensers, loss of optimum performance is encountered when any surface is covered by frost, or foreign material. Frosting, or the build-up of ice crystals, can cause an evaporator to become ineffective. Foreign material can render a condenser inefficient. It is difficult to maintain efficiency of a refrigeration system that cannot be kept free of frost accumulation in excessive amounts, or a condenser that is not kept open to the induced air current.
The general pattern for construction of heat transfer devices where the primary fluid is a liquid, and the secondary fluid is air, requires the air movement be perpendicular to the conduits containing the liquids, necessitating air movement over the largest dimension of the exchanger.